1. Field of the Invention
The present invention relates to a film-forming composition capable of forming a film having excellent dielectric properties, adhesion, film uniformity and mechanical strength, and having reduced moisture absorption; a preparation method of a porous film; a porous film formed thereby; and a semiconductor device which comprises the porous film inside.
2. Description of the Related Art
In the fabrication of a semiconductor integrated circuit, as the circuit is highly integrated, an increase in interconnect delay time due to an increase in interconnect capacitance, which is a parasitic capacitance between metal interconnections, disturbs the performance enhancement of the semiconductor circuit. The interconnect delay time is so-called RC delay which is in proportion to the product of the electric resistance of the metal interconnection and the electric capacitance between the interconnections. In order to minimize this interconnect delay time, the resistance of a metal interconnection or the interconnect capacitance has to be decreased.
By reducing the resistance of the interconnection metal and the interconnect capacitance, an interconnect delay can be lowered even in a highly integrated semiconductor device. This enables size reduction and speeding up of a semiconductor device. Moreover, power consumption can be minimized.
In recent years, metallic copper interconnections have been more popularly used in the structure of a semiconductor device than the conventional aluminum interconnections in order to reduce the resistance of metal interconnections. However, performance enhancement cannot be attained desirably only by the adoption of such a structure. A reduction in interconnect capacitance is an urgent necessity for obtaining a semiconductor device having a higher performance.
One method for minimizing interconnect capacitance is to reduce the dielectric constant (relative permittivity) of an interlayer insulating film disposed between metal interconnections. In order to reduce the dielectric constant of a material to about 2.5 or less, there is a method of introducing pores inside thereof to make it porous.
However, the material which has been made porous inevitably has lowered mechanical strength. This becomes a serious problem in the fabrication of a semiconductor device. In addition, lowering in the mechanical strength of a film leads to inadequate strength of the semiconductor device itself, causing a problem such as deterioration in the reliability of the semiconductor device.
Development of a material having both a low dielectric constant and high mechanical strength has come to be indispensable.
Methods for forming an interlayer insulating film known to date can be classified roughly into a chemical vapor deposition method and an application method. Each of these methods has its own merits and demerits. From the viewpoint of the mechanical strength, however, the chemical vapor deposition method is advantageous when a dielectric constant is about 2.6 or greater, while the application method is advantageous when a dielectric constant is about 2.6 or less.
Among various materials used in the application method, materials having a main skeleton of silicon oxide in which pores can be formed relatively easily compared with organic materials are expected as a next-generation insulating material.
When a conventional material is used in the application method, however, it is difficult to simultaneously satisfy a low dielectric constant and high mechanical strength necessary for the practical manufacture of a semiconductor device.
In the first example, it is described in Japanese Patent Application Unexamined Publication No. 9-315812/1997 that although a product obtained by hydrolytic condensation of an ordinarily employed silicon-based monomer cannot have an adequate strength, it is possible to form a film having a low dielectric constant which is excellent in adhesion with a coated surface, mechanical strength, chemical resistance and crack resistance by using a composition containing the reaction product between silica particles and hydrolytic condensation products of a mixture of alkoxysilane represented by XnSi(OR)4-n, and halogenated silane represented by XnSiX′4-n.
In the second example, according to the methods described in Japanese Patent Application Unexamined Publication Nos. 2001-294809, 2001-354904 and 2002-20688, a product obtained by the hydrolytic condensation of a silane compound in the presence of a basic compound is used in order to overcome the above problems. It is also described that a material prepared by these methods is made of a macromolecule and is not in the form of particles.
In the third example, according to Japanese Patent Application Unexamined Publication No. 9-315812/1997, disclosed is a film-forming coating solution, comprising silica particles in order to improve the mechanical strength of the film. In this method, a silica-based coating solution is obtained by the reaction between silica particles and a partial hydrolysate of alkoxysilane or halogenated silane on the silica surface.